1. Field of the Invention
The present invention relates for example to a punch for pressing a powder such as a rare-earth alloy powder into a compact, for manufacture of a magnet used in a voice coil motor, a powder pressing apparatus and a powder pressing method using the above punch, a compact formed by such a pressing method as above, a sintered body and a voice coil motor using the sintered body.
2. Description of the Related Art
Referring to FIG. 14 and FIG. 15, description will cover a conventional method for pressing the rare-earth alloy powder into a compact 8 (see FIG. 16). The compact 8 is sintered and then used as a magnet for the voice coil motor and so on.
In order to manufacture the compact 8, a tolling 1 for pressing operation as shown in FIG. 14 is used. The tolling 1 includes a die 3 having a through hole 2, a lower punch 4 to be inserted into the through hole 2 in advance, and an upper punch 5 to be inserted into the through hole 2. The lower punch 4 is provided with an upper surface having a center portion formed with a generally arcuate projection 4a and two edge portions each formed with a flange-like projections 4b. The upper punch 5 has a lower surface formed with a recess 5a. Each of the lower punch 4 and the upper punch 5 is made of a hard alloy such as cemented carbide, and for protection from chipping and cracking, each tip of the projection 4b and an edge portion 5b is chamfered by 0.8 mm.
When pressing, first, the lower punch 4 is lowered to form a cavity 6 in the through hole 2, and the cavity 6 is fed with a rare-earth alloy powder 7.
Then, the rare-earth alloy powder 7 in the cavity 6 is pressed between the lower punch 4 and the upper punch 5 while being oriented by a magnetic field. The pressing operation to the rare-earth alloy powder 7 is continued until the two edge portions 5b of the upper punch 5 are about to contact the corresponding projections 4b of the lower punch 4 (until a gap between the punches becomes about 1 mm for example) as shown in FIG. 15 in order to form a shape as close as of a final product.
As a result, a compact 8 as shown in FIG. 16 is obtained.
The compact 8 is formed to have a generally arcuate section, including an upper surface 8a formed by the recess 5a of the upper punch 5, a lower surface 8b formed by the projection 4a of the lower punch 4, slopes 8c formed by the projection 4b of the lower punch 4, and end surfaces 8d formed by a wall of the through hole 2.
As shown in FIG. 16, the compact 8 has a problem that cracks A develop along border portions 8e between the upper surface 8a and the slope 8c. 
Causes of the crack A will be described.
As shown in FIG. 14, when the rare-earth alloy powder 7 is fed into the cavity 6, layers of a marking material B having a color different from that of the rare-earth alloy powder 7 were inserted at a predetermined interval, and then the pressing operation was made. Then, as shown in FIG. 15, the gap between layers of the marking material B was found to be very narrow between the projection 4b of the lower punch 4 and the edge portion 5b of the upper punch 5. This indicates that the rare-earth alloy powder 7 has a much higher density between the projection 4b of the lower punch 4 and the corresponding edge portion 5b of the upper punch 5 than in other portions. This is presumably that when being pressed, the rare-earth alloy powder having a poor flowability was sandwiched between a tip of the projection 4b of the lower punch 4 and a tip of the edge portion 5b of the upper punch 5, became unable to move, and was compressed into such a high density at the border portion 8e. Therefore, pressure exerted to the compact 8 is relieved when the compact 8 is taken out of the through hole 2 of the die 3, allowing a highly compressed portion such as the border portion 8e to expand more significantly, often developing cracks or fractures. A similar problem occurs in a sintering process.
If the rare-earth alloy powder 7 is pressed in a strong magnetic field not smaller than 0.5 MA/m oriented in a direction indicated by an arrow C (longitudinally of the projection 4b) shown in FIG. 16, grains of the magnetized rare-earth alloy powder 7 repel against each other, making the powder density higher in a region closer to a perimeter of the cavity 6 than in a center portion of the cavity 6, increasing further the density near the projection 4b. Further, if the rare-earth alloy powder 7 fed into the cavity 6 is wiped flush at an upper edge of the cavity 6 by a lower edge of feeder box (not shown), region to be pressed by the projection 4b is fed with an excess amount of the rare-earth alloy powder 7 than needed. Since the rare-earth alloy powder 7 does not have enough flowability, the density in this region after the pressing becomes higher than in other regions. Therefore, in these cases, the compact 8 becomes more apt to develop the cracks or fractures in the border portion 8e when taken out of the cavity 6.
It is therefore a primary object of the present invention to provide a punch, a powder pressing apparatus and a powder pressing method capable of preventing the cracks and fractures from developing in the product, thereby improving productivity.
Another object of the present invention is to provide a compact manufactured by the above method.
Still another object of the present invention is to provide a sintered body and a voice coil motor using the sintered body.
According to an aspect of the present invention, there is provided a punch used for pressing a rare-earth alloy powder, comprising a punching surface for pressing the rare-earth alloy powder. The punching surface has an edge portion including a projection having a tip chamfered by a width not greater than 0.5 mm.
According to another aspect of the present invention, there is provided a powder pressing apparatus comprising the above described punch having the tip chamfered by the width not greater than 0.5 mm, and a die having a through hole for insertion by the punch.
According to another aspect of the present invention, there is provided a powder pressing method using the above punch having the tip chamfered by the width not greater than 0.5 mm and a die having a through hole for insertion by the punch. The method comprises a first step of feeding a rare-earth alloy powder into a cavity formed in the through hole, and a second step of pressing the rare-earth alloy powder fed into the cavity, by using the punch.
According to still another aspect of the present invention, there is provided a compact manufactured by the above described powder pressing method.
The rare-earth alloy powder has a sharp-edged grain, and has poor flowability. Therefore, even if the rare-earth alloy powder is vibrated for example at the time of pressing, the rare-earth alloy powder cannot move smoothly in the cavity during the pressing, making difficult to achieve uniformity of the compact density. However, according to the present invention, by decreasing the width of the chamfered portion, which is a portion formed at the tip of the projection provided at the edge portion of the punching surface, to 0.5 mm or smaller, thereby reducing the amount of the rare-earth alloy powder pressed and clogged by the edge portions, the flowability of the rare-earth alloy powder around the edge portion is improved. Therefore, when a pressing force is applied to the rare-earth alloy powder at the tip of the projection of the punching surface at the time of pressing, the rare-earth alloy powder moves to a region of a lower density along the slope of the projection, without stagnating at the tip of the projection. Therefore, it becomes possible to obtain a compact having a uniform density, making possible to prevent the cracks and fractures caused by the non-uniform density.
According to an aspect of the present invention, there is provided a punch used for pressing a rare-earth alloy powder, comprising a punching surface for pressing the rare-earth alloy powder. The punching surface includes a projection, and the projection includes a slope having a surface roughness Ra not greater than 1.0 xcexcm.
According to another aspect of the present invention, there is provided a powder pressing apparatus comprising the above described punch provided with the slope having the surface roughness Ra not greater than 1.0 xcexcm and a die having a through hole for insertion by the punch.
According to another aspect of the present invention, there is provided a powder pressing method using the above punch provided with the slope having the surface roughness Ra not greater than 1.0 xcexcm and a die having a through hole for insertion by the punch. The method comprises a first step of feeding a rare-earth alloy powder into a cavity formed in the through hole, and a second step of pressing the rare-earth alloy powder fed into the cavity, by using the punch.
According to still another aspect of the present invention, there is provided a compact manufactured by the above described powder pressing method.
According to the present invention, by making the slope roughness Ra not greater than 1.0 xcexcm, it becomes possible to increase the flowability of the rare-earth alloy powder at the time of pressing. Therefore, the rare-earth alloy powder being in a high-density region such as around the tip of the projection of the punching surface, moves along the slope to a low-density region. As a result, it becomes possible to increase uniformity in the density of the rare-earth alloy powder within the cavity. Therefore, it becomes possible to obtain the compact having a high uniformity of the density, making possible to prevent the cracks and fractures caused by the non-uniform density.
Preferably, the punching surface is made of an alloy steel or a hard alloy (cemented carbide). In this case, abrasion resistance of the punching surface can be improved. Therefore, even if the chamfering width of the tip of the projection is not greater than 0.5 mm or the surface roughness Ra of the punching surface is not greater than 1.0 xcexcm for increased flowability of the rare-earth alloy powder, the punching surface is virtually free from wear, making possible to keep the good pressing.
Further, preferably, at least the projection in the punching surface has an HRA hardness not smaller than 75 and not greater than 93. In this case, even if the projection of the punching surface is machined into a sharp edge, the punch can be used for a long period of time without failure in the tip of the projection because of improved toughness.
The flowability becomes even more decreased if the rare-earth alloy powder is added with a lubricant. However, since the flowability of the rare-earth alloy powder can be increased according to the present invention, the uniformity of the powder density within the cavity can be increased even if the lubricant is added.
If the rare-earth alloy powder is manufactured by a quenching method, the flowability becomes extremely poor, since grain size distribution curve of the rare-earth alloy powder becomes sharp, with the grain size confined in a small range. However, since the flowability of the rare-earth alloy powder can be improved according to the present invention, the powder density uniformity within the cavity can be improved even if the rare-earth alloy powder is made by the quenching method.
Preferably, a compact after the pressing has a density of 3.90 g/cm3xcx9c4.60 g/cm3. In this case, necessary strength of the compact can be achieved, and a rare-earth magnet having a good magnetic characteristic can be obtained.
At the time of pressing, if the magnetic field is applied to the rare-earth alloy powder in the cavity, in a direction vertical to a direction of pressing by the punch, grains of the magnetized rare-earth alloy powder repel against each other, and the powder density tends to become higher in an region closer to a perimeter of the cavity than in a center region of the cavity. However, since the flowability of the rare-earth alloy powder on the punching surface can be improved according to the above invention, the rare-earth alloy powder in a high-density region moves to a low-density region. Therefore, difference in density of the rare-earth alloy powder within the cavity can be decreased even if the rare-earth alloy powder is oriented in the magnetic field.
Further, if the rare-earth alloy powder is oriented in the magnetic field not smaller than 0.5 MA/m, the repelling force among the magnetized rare-earth alloy grains becomes larger, making the powder density distribution within the cavity non-uniform, making the density higher in a region closer to a perimeter of the cavity. However, according to the present invention, even if the magnetic orientation is made as above, the non-uniformity of density within the cavity and the increase in density near the perimeter of the cavity can be reduced, making possible to effectively reduce the cracks and fractures.
According to another aspect of the present invention, there is provided a powder pressing method using a tolling including a die having a through hole, and a pair of upper and lower punches. At least one of the upper and lower punches has a punching surface for pressing the rare-earth alloy powder, and the punching surface has an edge portion provided with a projection. The method comprises a first step of feeding the rare-earth alloy powder into a cavity formed in the through hole, and a second step of pressing the rare-earth alloy powder fed in the cavity, by using the upper and lower punches. In the second step, the upper punch and the lower punch are brought closest to each other at a minimum distance not smaller than 1.7 mm.
According to the present invention, the compact after the pressing can have a side surface of a width not smaller than 1.7 mm. Therefore, the cracks and fractures in the side surface of the compact can be reduced.
According to another aspect of the present invention, there is provided a compact made of a rare-earth alloy powder, comprising: a main surface formed convex; another main surface formed concave; a slope from an edge of said another main surface; and a side surface formed between said main surface and the slope. According to this compact, the side surface has a width not smaller than 1.7 mm. Or, said main surface has a maximum height H, the side surface has a width S, and S/H is not smaller than 0.15.
According to the present invention, the width of the side surface of the compact is not smaller than 1.7 mm. Or, the width of the side surface of the compact has a ratio not smaller than 0.15 to the height to the highest portion of said main surface. Therefore, it becomes possible to prevent extreme increase in the density in the side surface, making possible to reduce the difference in the density from other portions. As a result, the crack and fracture in the side surface of the compact can be reduced.
According to another aspect of the present invention, there is provided a sintered body made of a rare-earth alloy powder, comprising: a main surface formed convex; another main surface formed concave; a slope from an edge of said another main surface; and a side surface formed between said main surface and the slope. According to this sintered body, the side surface has a width not smaller than 1.45 mm. Or, said main surface has a maximum height H, the side surface has a width S, and S/H is not smaller than 0.15.
In this case, the compact at the time of the pressing is formed to have a width of a side surface not smaller than 1.7 mm. Or, the width of the side surface of the compact at the time of the pressing is formed to have a ratio not smaller than 0.15 to the height to the highest portion of said main surface. Therefore, the cracks and fractures in the side surface of the compact can be reduced. As a result, yield in the manufacturing process can be improved, and productivity of the sintered body can be improved.
According to another aspect of the present invention, there is provided a voice coil motor using the above sintered body.
The sintered body obtained as described above is less apt to develop such a failure as cracks and fractures. Therefore, by using such a sintered body, a voice coil motor of a stable quality can be obtained.